Hi!
This might sound silly, but I think I have found the reason (not the solution yet). I must tell, that I’ve never ever in my life seen a faster WordPress, than yours. Basically the results are there, but the javascript can’t syncronize the values with the animation (the ajax request is coming in under 300 milliseconds, which is 2.5 times faster of what I have ever seen)
I will try to change some values on my local server, to see if I can reproduce this, and hopefully post the solution in a couple of hours.